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Boeing 737 takeoff config question
On the Boeing 737, what's the difference between taking off in LEVEL CHANGE or no vertical mode selected (i.e. both LEVEL CHANGE and VNAV button blank on MCP).
I realize in both the MCP speed window will open and the PM will have to manually bump up the speeds at the appropriate time.
Normally we almost always takeoff in VNAV mode.
Some might be wondering why would one take off in non VNAV mode. In VNAV mode you're normally climbing at V2+20 to acceleration height.
If you're in a area with high terrain, you can climb in non-VNAV mode and climb out at V2 or less than V2+20 for a height rate of climb. When reaching accel ht., press VNAV or LEVEL CHG.

Before addressing the specific questions, some background on how the autoflight system works:
It's assumed that the pilot presses TO/GA when beginning the takeoff roll.  This will clear any MCP modes such as heading or approach or level change or vertical spped if they were still engaged, but in the case of LNAV and VNAV those will remain armed to engage at the appropriate time (if the pilot had armed them already by pressing the button on the MCP).
(Not part of the question, but just for completeness, LNAV will engage, if armed, at 50' AGL after takeoff and initially command a straight-ahead track, commanding a turn as necessary to meet the loaded departure procedure.  If the initial track in the LEGS page doesn't align closely with the departure runway, LNAV can't be armed on the ground.)
If armed, VNAV will engage at 400' and command V2 + 20 until the acceleration altitude (typically 1000' above the runway), at which time it will command 230 knots or the maximum airspeed for the current flap configuration, if less.  And as the flaps are retracted, that commanded speed will increase, eventually matching the FMC climb speed schedule - typically, 250 knots until 10,000', then the ECON climb speed in IAS until transitioning to the ECON mach number climb speed.
If VNAV was not armed, then the TOGA mode will remain engaged indefinitely; there is no fixed altitude at which it will change over to something else.  Per se, TOGA commands wings level and V2+20.  At autopilot engagement, that changes over to Level Change and the window opens at the current speed.  And of course if the pilot selects level change, then the window opens and that - along with Heading Select (or LNAV, if already active) - is the mode you're in.  Any of those can be selected at or above 400' RA.
And, if the vertical mode goes into Altitude acquire, then the window opens at the current speed.  Same if Vertical Speed is selected, the window opens at the current speed.
So for the scenarios asked about in the first paragraph of the question,

I've never seen "taking off in level change" done in the aircraft.  I suppose you might be able to engage the mode on the runway (along with LNAV or Heading Select), but it's not something I've ever seen done so I can't tell you how it would actually work. (You certainly wouldn't have autothrottles setting takeoff thrust for you, for one thing.)
If you take off with nothing armed and without pressing TOGA, you simply have no flight directors or other automation engaged, and the speed window will remain at V2 until it's changed.
If you had Level Change engaged but then press TOGA, Level Change is gone & you have TOGA vertical guidance.
If you have nothing armed and press TOGA at the start of takeoff, then TOGA vertical guidance will stay engaged until some other vertical mode engages (either VNAV at 400', altitude acquire, or by a pilot pushing a button).

In your fifth paragraph, you wrote that TOGA will command wings level and V2+20. Sure about that? I thought only VNAV did that automatically with the speed window closing. I thought in non-VNAV mode we had to roll the speed window up manually at 400 and clean maneuv. speed at acceleration height. But thanks on the answer to my question. Taking off in LVL CHANGE and pressing TOGA cancels LVL CHNGE and is same as no vertical mode right?

Yes, wings level and V2+20 is correct. If you're showing TOGA in the FMA, then that is what your vertical guidance will be (unless in windshear) on departure. TOGA very much is a vertical mode.

My question was the speed window - will it go from V2 to V2+20 at 400 agl. I'm thinking it won't. Has to be done manually. I'll test it out on the real thing and report back.

With TOGA (and nothing else) active, the window will show the set V2, both below and above 400'. The flight director guidance will be to hold V2+20.

Interesting - I always thought that the FD showed V2 till 400 agl and then V2+20 w/ speed window closed (VNAV mode). So you're saying even though the window shows V2 - FD will guide V2+20 on takeoff. In TOGA, it'll maintain that until you select another mode (e.g. LVL CHG). I wonder what happens if at accel height, you just roll the speed window up to clean manuv speed w/o selecting another vert mode - will the FD command the new speed now??? As far as engine out is concerned on t/o - my understanding is that it'll be from V2 to V2+20 depending on active speed.

Yes, engine out has its own logic that's separate from the normal discussions. There, it'll command no more than V2 + 20 (or +15 in some versions, IIRC), no less than V2, and "existing speed at failure" if between those two.  Been a while since I've dialed in a new speed at accel height, but I think that's correct, that it then acts more or less like a "level change" with the new speed.  Letting VNAV do its thing is easier & less time staring at the MCP! Below 400', TOGA (and VNAV) most certainly DO NOT command V2 -- that's too slow for comfort on a normal profile! (Nor above 400' either)


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Evaluating maximum TAS for a given available power
This is the original problem:

A turboprop aircraft with a weight of 162 kN is equipped with a power plant with a power derating model described by $\frac{P_A}{P_{A,0}} = 0.9\sigma^{0.75}$, where $P_{A,0}$=3.5 MW is the sea level available power. The aircraft has the following aerodynamic parameters: $C_{D,0}$=0.018, wing area S=52 m2, wingspan b=24 m and efficiency factor e = 0.82. Determine the maximum true airspeed at an altitude of 6 km.

I evaluated the available power to be 1.981 MW in an earlier part of the question. By solving $\sqrt{\frac{2W^3(C_{D,0}+KC_L^2)^2}{\rho{S}C_L^3}}=1.981\times10^6$ for $C_L$, I found that $C_L$ could either be .3057 or 12.82. I chose to use $C_L = .3057$ in solving $\frac{2W}{\rho{S}V^2} = C_L$ for $V$ to find the maximum TAS.
Aside from being ridiculously large, would there be a better explanation for why I should disregard $C_L = 12.82$?

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Reference area in lift coefficient calculation
Does the reference area changes with angle of attack?
Most sources says it is projected area. If it is projected area then it should depend on Angle of attack.
I want to calculate Lift and drag coefficient from a CFD simulation. I have got the Lift and drag force.

That it is the projected and not the actual area is meant to eliminate dihedral. Then use the projection at AoA = 0°.

No, it does not. It does not change even if the actual wing area does e.g. by extending Fowler flaps.
Lift is only very roughly proportional to wing area. It depends on the span and the chord and the profile in complex ways. By referencing to the wing area you are just getting a number that is of the same order for different wings so you can more easily see how good they are.
Referencing to different area for different angle of attack or different flap positions would just hide part of the effect of those on lift, so you reference to the same area for all flight regimes to have all the effect show in the lift coefficient.

The idea of a coefficient is precisely that it contains all those factors likene geometry, angle of attack factors etc, and

@CarlBerger, those factors are not separable from size. If you take a wing and scale it to double the area, it will not have the same coefficient of lift, nor the same critical angle of attack.

my post was sent before ready. sorry.
concerning the coefficients.. by definition: Lift = CL * p_Dyn * A,
hence double the area means double the lift, when at the same lift coefficient.
What you're trying to say is that e.g. lift coefficient is not constant when applying sheer geometric scaling, but noone questioned that. CL is anyway depending on lots of things (angle of attack, compressibility, ..)
And yes to answer op's question, A remains constant.

@CarlBerger, you kind of contradict yourself – since the coefficient does not stay the same when you apply scaling, double the area does not in practice mean double the lift.

The reference area is arbitrary, i.e. you can choose what your reference area is. But the idea of a reference area is that it is constant, and the influence of e.g. angle of attack on lift is reflected in the coefficient of lift. Some form of projected wing area is just the most common choice for airplanes (and there are certainly some good engineering reasons for choosing it).
If you compare drag coefficients for cars and airplanes, the former are usually based on frontal area, whereas the latter are based on what is roughly the wing area (typically including part of the fuselage), so you can't even compare dragginess between the two types of vehicles by just looking at the drag coefficient.

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Do airliners reduce their angle of attack to maintain lift = weight as fuel burns off during cruise?
For an aircraft to remain in level flight, it must generate lift force that is exactly equivalent to its weight. However, modern airliners lose a lot of weight when they are in cruise flight in the form of fuel burning off. Therefore, it stands to reason that to remain in level cruise while the aircraft's weight reduces, the lift the wing generates must also reduce. How do airliners accomplish this? They generally don't adjust their cruising speed, and the wing geometry itself doesn't change (purposefully), so I assume that airliners fly at reduced angle of attack to remain level?
Edit: It's been made clear in the comments that there are multiple ways this could be accomplished. I'm curious how exactly it's done by airliners today.

AoA must be reduced, which will reduce drag. The real question is whether they reduce throttle or speed up.

Why not hold a-o-a constant at the most efficient value, and throttle back as needed to maintain altitude, accepting the resulting reduction in cruise speed?

Not to confuse this with orbital mechanics, but they can go higher and slow down their IAS.  This would keep their TAS more "on schedule" and maintain optimal AoA.

Does this answer your question?

RE "and throttle back as needed to maintain altitude, accepting the resulting reduction in cruise speed?" -- or, if permission is obtained, leave the throttle setting fixed and use the excess thrust to climb-- as several answers have suggested-- since lift is nearly equal to weight regardless of whether the aircraft is climbing or flying level, and is in fact slightly LESS than weight in a climb, this method too must involve a DECREASE in airspeed, if a-o-a is left constant and a/c is not meant to follow a looping trajectory!

Yes.  As fuel is burned the airplane will want to climb, so the autopilot (which is flying the plane 99% of the time and will normally be in altitude hold mode once you're at your clearance Flight Level) will reduce pitch attitude and AOA ever so slowly as weight and lift required is reduced.
Without autothrottle, if you leave the thrust alone and are paying close attention, you will also notice the Mach# creep up slightly and may tweak the thrust back eventually to reduce it, to keep speed constant if that is necessary, taking the bonus reduction in fuel burn.
If the plane has an autothrottle system, the autopilot will be maintaining both the flight level and the airspeed. So if you are just sitting there watching it carefully over time, you should observe the pitch attitude slowly decrease just about imperceptibly, and the thrust setting also decreasing almost imperceptibly to maintain speed/mach, with the fuel burn going down.

It's almost always more economical to fly higher if you can. So, as an airliner burns off its fuel load, it will naturally want to climb higher, and the pilot will request clearance to do so in stages as the gross weight of the plane goes down.
If you can't get clearance for more altitude then you trim off the AoA to maintain constant altitude, and you fly a little faster.

The change is (of course) continuous, so there is constant minute necessity for trimming. Pilots rarely do this, as autopilots are controlling airliners most of the time.

There is an interesting nuance here that I'm sure has not been fully addressed by this answer.  If a-o-a is held constant, and weight decreases, airspeed must decrease, or else the excess airspeed will cause the lift force to be greater than weight, in which case the trajectory will resemble the entry to a loop.  If a-o-a is held constant, and thrust is not reduced as weight is reduced, and the aircraft is allowed to climb as a result, there will be some decrease in airspeed.  To a first approximation, the decrease will be proportional to the square root of the change in wing loading.

Only "to a first approximation", because this is ignoring the fact that lift actually is slightly less in a steady-state climb than in level flight.  So the actual decrease in airspeed must be slightly greater than described above.  If airspeed is held constant and airplane is allowed to climb as weight decreases and thrust is held constant, you can be sure that the angle-of-attack has decreased.

There was an 'exception' which demonstrated the effect more naturally: Concorde. Its design cruise mode was Mach hold with altitude, and it was gradually climbing as it got lighter (or as air got colder, etc.) It was normally cleared for all levels above.


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Comparisons between the A340 and 747-200 wing: sweep angle and span
Here is a picture of the wings of the A340 (launched in the late 1980s) and the Boeing 747-200/400 (launched in the early 1970s / 80s) superimposed upon each other (source: https://www.amazon.com/Evolution-Airliner-Ray-Whitford/dp/186126870X).

I was hoping to use this image to inquire about technological improvements in wing design between 1970 and 1990. The reason for the comparison between these two airliners in particular is that they seem as close to a "fair" comparison that can be made between airliners over very different time periods, specifically because they're designed to carry relatively the same amount of people.
Specifically, there were two facts about the wing planform that I wanted to ask about.

Greater aspect ratio. The A340 has a much greater aspect ratio because it has the same span but much lower total wing area. This improved the L/D ratio by reducing induced drag. What allowed engineers to use a higher aspect ratio wing for the A340? I.e., could the engineers at Boeing in the late-1960s have designed a plane with the same aspect ratio as the A340 if they wanted to (in which case, the higher aspect ratio on the A340 was a design choice, and not a result of any improvement in techniques, methods, materials etc.).

Less wing sweep. Wing sweep is desirable because it lets you fly faster without incurring extremely large drag penalties (increases critical mach number), but it does so at the expense of lift. Therefore, if you can get away with a lower sweep angle for a given speed, that's something that would be good for fuel efficiency. How were engineers able to get away with reducing the wing sweep of the A340 while having it fly at generally speaking, the same cruise speed as the Boeing 747?

Thanks so much!

The A340 has a much greater aspect ratio because it has the same span but much lower total wing area. This improved the L/D ratio by reducing induced drag. Sorry, but wrong. Induced drag is unchanged, what is lowered is viscous drag.

Note that the wing on the early A340-200 and A340-300 is a different design than used on the later A340-500/600. I don't think it makes a big difference for your question, but there are in fact two A340 wing designs.

Both less wing sweep and increased aspect ratio are accommodated by the progress in application of rear loading supercritical wing profiles in the couple of decades between design of the two types.

Pic above is from Torenbeek, earlier used in this answer, and depicts the increased thickness of the supercritical profile - which enables higher aspect ratio. But the main difference is actually the higher allowed Mach over the wing => less wing sweep required => way less constructive and aerodynamic problems.
Aspect Ratio is $b^2/S$. Start sweeping the angle of any given wing and the span reduces, so there is a feed-forward loop in the two parameters considered in the question.

Thanks! That makes sense to me. A follow-up question if you don't mind. As far as I'm aware, the first supercritical sections were introduced on the generation of aircraft launched in the late70s - early 80s (767, 757, A300, A310). Did the aircraft of the late-80s - early 90s (A340, 330, 777) incorporate "improvements" to their supercritical sections? If so, what specific improvements were made?

Yeah that's a hard one, the manufacturers keep that information close to their chest. I don't have any academic research docs from that period.

@interested22 please ask your follow up question as an actual question. There's no place to answer it down here. Plus, while Koyovis might not have info, others might and they might not see your question down here in the comments.

When development of the A330 / A340 started, those designs were internally called TA9 and TA11, TA standing for Twin Aisle. A study (sorry, no link, this was before the WWW) by Airbus Bremen showed marginal advantages for a high aspect ratio wing but a wing of lower aspect ratio was chosen eventually. Why? It offered more internal volume for fuel and a simpler flap mechanism.
Still, the new wing has a higher aspect ratio compared to earlier airliners simply because less fuel needs to be carried along with the low specific fuel consumption of modern high-bypass turbofans, even for a record-setting long range flight from Paris to New Zealand.

Even if the MTOWs are similar, the comparison might be anyway a bit unfair because the wing of the A340 was developed  for the A330 as well, so most probably some additional compromises were imposed during its design.

First of all a short clarification in terms:

induced drag is proportional to wingspan, not aspect ratio.
induced drag coefficient is proportional to aspect ratio.

In fact, $C_{D_i}$ is normally expressed as:
$C_{D_i}= \frac{C²_L}{\pi AR e}$
But the relevant drag term is:
$D_i = qS \cdot C_{D_i} = qS \frac{C²_l}{\pi eAR} = qS \frac{L²/q²S²}{\pi eAR}=\frac{L²/qS}{\pi eb²/S}=\frac{L²}{q \pi e b²}=\frac{W²}{q \pi e b²}$
Where $W$ is the weight, $b$ the wingspan, $e$ the Oswald coefficient and $q=½ \rho V²$.
So induced drag is proportional to the wingspan $b²$. That means that being $b$ the same between B747 and A340, then $D_i$ is also the same for both of them (obviously if $W$ and $e$ are the same).

So the real question now actually becomes what drives the choice of $b$ (and not $AR$) during design phase? Looking at the previous equation of $D_i$, one can see that it increases with the weight and decreases with the speed. That means that $D_i$ is maximum when the weight is big and the speed is small i.e. at takeoff. And indeed, normally for a jetliner the requirement on the OEI climb at take off is the sizing requirement for $b$. That's also why an airplane usually gets a wider wing as soon as the weight increases due to some update.
So, having sized the wingspan, once that also the value of $S$ is chosen satisfying all the necessary requirements, the $AR$ that pops out is the one that is needed. But this triggers then the next question.

Why has the B747 a bigger wing surface than the A340? Generally speaking, a bigger surface is never good for something flying because it implies bigger drag. But a wing with a bigger $S$ does not generate only more drag but more lift as well. And, again, a lot of lift is needed at takeoff... especially if the available engine(s) is not pushing that much, as was the case for the B747: the original Pratt & Whitney JT9D delivered some 200kN thrust, it was the very first commercial turbofan and it was the non plus ultra of those times. A modern Rolls-Royce Trent 900 as seen on the A380 can easily push almost twice as much. So for the original B747 a big wing surface was needed to compensate for the lack of thrust and meet anyway the takeoff requirements. This is also demonstrated by the very complicated slats and flaps used on its wing. And, last but not least, bigger wing gives also more volume for fuel, which is something that the original JT9D for sure needed.

Last topic: what about wing sweep? Here the choice is a bit more complicated because sweep angle is mutually influenced by airfoil, taper ratio, twist and possibly other aerodynamic criteria.
Sweep is mainly used to increase (increase in this case is good) the critical Mach number but it also improves lateral stability and shifts the CG forward improving pitch stability as well. All this happens at the expense of an higher structural weight and possibly pitch-up problems if combined with an high AR (so one of the two should be low).
An higher sweep allow the use of a somewhat thicker airfoil giving a more efficient wing structure, partially compensating for the higher weight due to the sweep itself. And thicker airfoils also allow more fuel volume.

As usual in the aerospace world, it is a matter of compromises among contrasting requirements.


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If sweep angles are high enough can supersonic wings use thick, rounded airfoils?
In lectures on aerodynamics, it's always emphasized that the effect of sweep is to lower the Mach number normal to the leading edge. In the context of supersonic flow, this ensure that the leading edge of the wing is experiencing a normal Mach number that is subsonic and thus is a "subsonic" leading edge. See this image from W.H. Mason's notes.

However, the question I run into is, assuming I have picked a sweep angle that appropriately ensures that the normal Mach is not just < 1 but also < M_cr of a relevant airfoil,  why can't we just use normal transonic airfoils of transport t/c (e.g., 10%) and perform aerodynamic analysis that is exactly like what you would do for a transonic transport aircraft traveling at M 0.8?
So for example, let's say I want to cruise at Mach 1.5, and I pick a sweep angle of 60 degrees, since cos(60)* 1.5 = 0.75, the "normal" Mach number is 0.75. Therefore,  I would have thought that I could use a NASA supercritical airfoil (which has a M_dd of > 0.75) and still have the transonic airfoil operate in a regime that is identical to if we were using the airfoil for a transport application. For example, see this example airfoil from the NASA supercritical publication. In particular, there should be very little wave drag on my wing here because the normal Mach is operating well below the M_dd of the airfoil.

But of course, I am clearly wrong. When I actually see airfoils for supersonic transports, they never do anything like what I said in the last paragraph. They are almost all very thin (t/c < 4%) and are much less rounded than subsonic airfoils, and this is true even when leading edges are highly swept.

OK, I figured out the answer. In essence, I am ignoring 2 drag components which only exist in supersonic flow.
In subsonic / transonic flow you have the following drag components: profile drag (which has a friction component and a pressure component called 'form' drag) and "induced" drag due to lift (aka"vortex"drag). In transonic flow, we add a new component, which is the wave drag from the shockwaves which form due to local acceleration of air beyond sonic velocity as it slows down. Notably, this wave drag component is a "near field" phenomenon in that it depends on the pressure distribution near the airfoil.
However, in supersonic flow, in ADDITION to all those components, you have two new "far field" components which are analagous to the "vortex" drag in subsonic flow: wave drag due to lift and wave drag due to volume. This wave drag is also due to shock wave formation, but NOT the same shockwaves we talk about in transonic flow, it is due to the need to push the air radially outward. This wave drag, specifically that due to volume, is ~ to t/c. Therefore, while sweeping the wing will help you control the local shockwave formation, you cannot use high t/c airfoils because it will adversely effect your far-field drag components.
TY to the Boeing aerodynamics fellow who helped me.

Not quite - wave drag is mostly pressure drag: Overpressure on forward-facing surfaces and suction on back-facing surfaces. Make the parts to which those surfaces belong thinner, and the pressure will have less of a horizontal component.

I'm a bit peaky but I'd call the additional drag at transonic speed simply "transonic drag" in order to keep it distinct from the proper wave drag that exists only at supersonic speeds. The transonic drag isn't only a pressure drag but also a viscous drag since it rises from the interaction between shock waves and boundary layer. Regarding the proper wave drag: I like the simpler explanation of @PeterKämpf here above.

You aren't entirely wrong.
Look at various fighters.  The F-5, F-16, and F-15 all use NACA 65A and 64A series.  The F-4 uses NACA 4-digit foils.  So all foils with rounded leading edges.
Likewise, most of the rest of the Century Series fighters use NACA airfoils with round LE (still very thin).
The thickness is still very thin -- typically 3-4% for each of these.
Supersonic planforms typically have low aspect ratio, sometimes all the way to a delta wing.  This gives the advantage of a large root chord, so that small t/c is multiplied by a large chord to give some dimensional thickness for the structures.  But rest assured, fighter wings are built very solid.
The XB-70 uses a 2% to 2.5% modified hex airfoil -- i.e. a truly sharp LE and TE.
The F-104 uses a 3.36% thick modified bi-convex airfoil -- another shape with a truly sharp LE and TE.
It depends on how fast you need to go and how much of your time you are going to spend there.
The Concorde and XB-70 spend almost all of their time at very high speed.
Fighters spend more time at a variety of speeds.  They only go their fastest during a dash.  Even for supersonic fighters, most aerial combat is done subsonically.  They need maneuverability -- which is helped with rounded leading edges.

Hm. But why so thin? If I want to travel at M1.5 and use 60 degrees of sweep, such that M_normal is 0.75, then I could get away with using a 10% t/c airfoil with M_cr > 0.75. Wave drag even with 10% thick airfoil should be low because we are < M_cr for the airfoil.

Drag divergence isn't really wave drag.  Drag divergence is a jump in drag caused by shocks on the upper/lower surface of an airfoil causing separation.  Wave drag is a separate phenemona.  You'll notice that many object's drag coefficient goes back down after it gets slightly supersonic.  Not as low as subsonic, but lower than transonic.  The thin wing will always be important to keep wave drag down.  Wave drag is about how much volume you're shoving through the air.


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How has Computational Fluid Dynamics improved commercial airliner wing design?
Starting the 1980s, extremely powerful computational fluid dynamics software has been used to design wing sections and whole wings. I am going to focus on the Boeing 777 as an example of CFD applied to commercial airliner design since this is the aircraft that Boeing itself claims had an enormous use of CFD in design of the wing and wing-airframe integration (source). To quote, "The 777, being a new design, allowed designers substantial freedom to exploit the advances in CFD and aerodynamics. High-speed cruise wing design and propulsion/airframe integration consumed the bulk of the CFD applications". In particular, "inverse design" was first used (at Boeing at least) for the design of the 777.
Conceptually, CFD is a tool in the process of wing design, a tool that must improve attributes of the wing that ultimately then result in  improvements in parameters which customers (airliners) care about. For example, one imagine the following causal chain: better CFD predictions of wing pressures --> some changed wing characteristics --> lower drag for a given lift --> lower fuel burn (which airlines care about).
My question centers around the "changed wing characteristics" part of the casual chain above. That is, I'm unsure as to what specific wing characteristics changed as a result of the application of CFD in the process of wing design. This could be changes in wing planform, thickness, t/c tapering, twist, or something else entirely. Basically, there has to be some output of the application of CFD that shows up in the actual wing itself to make the wing more efficient, and I'm curious what that output was.
Note: edited for clarity.

"What I am unclear about is how the use of advanced CFD software actually changed the wing characteristics", "I'm more interested in what actual properties of the wing changed through the use of CFD." What is your actual question? These two would require very different answers.

Sorry, but I don't understand how those two sentences are different :/. To be clear, I am interested in what physical characteristics of wings designed with intensive CFD applications changed relative to  wings designed without the use of CFD that were responsible for improved L/D and thus reduced fuel burn of the former relative to the latter (eg, wing gemoetry, wing twist, t/c ratio tapering, planform changes, etc.) Does that clarify? Can also edit main question!

I'm unsure as to what specific wing characteristics changed as a result of the application of CFD in the process of wing design.

Look at the change in airfoils over span of a swept wing. A simple swept wing of constant cross section would show a marked drop in lift around its center ("Mitteneffekt") and needs to be modified in order to straighten the isobars and use all of it to its fullest potential, something that is near impossible without CFD.
Another example is the optimization of the high lift configuration of a wing. This article explains how the flap system of modern airliners has been simplified over the last decades by use of CFD tools which had previously been unavailable.

Sequence of the CFD-based high-lift design process, Figure 9 from AERODYNAMIC DESIGN OF AIRBUS HIGH-LIFT WINGS IN A MULTIDISCIPLINARY ENVIRONMENT.
In case of the Boeing 777 the huge GE90 engines had to be fitted to a wing similar to what had been done on the 737 and 767 before. Interactions between nacelle and wing had to be considered from low speed, high lift flight to high Mach cruise, so compromises had to be found. Doing this with CFD made that task immensely easier.

Factors influencing nacelle installation design, copied shamelessly from the cited paper by Dennis Berry.

For the last twenty five years, at least, every significant development in aerodynamics has involved computational fluid dynamics.
Nobody has designed an aircraft, a wing, or even a car in the last twenty-five years without CFD being involved. (There may be some exception for some home-built ultralight somewhere in the world.)
The most specific advantage of CFD has been to reduce the cycle time of trying new things. So we can see what happens when we make a wing thicker - or thinner, or wider, or longer, or whatever. If we find that thinner wings work better in our case then we build thinner wings. Sometimes we check it by building an actual model, but in many cases we don't. In that sense we have learned what wings or fuselages or ailerons or wing mirrors (yes, car mirrors get tested by CFD) work better by using CFD.
So if you ask what we have learned from CFD, that is the same as asking what we have learned about aerodynamics. Other tools will have been involved, but CFD will have played a part.
(Incidentally learning "how to make wings thicker" is much more about structural mechanics than it is about fluid dynamics, computational or otherwise).

Thanks! Definitely recognize that CFD is now essential and that it has allowed wing designers to reduce their cycle times and try new things. I'm  curious about whether there is any identifiable trend in the things that were tried that ended up working and thus made wings more efficient over time. Maybe there isn't, and so there is no way to uniformly characterize the physical output of the application of CFD. But I'm curious if there is a trend. Does that clarify the question?

One of the huge advantages of CFD is that you can try many more things. So lots of the things that were tried were tried because CFD let us do it (rather than having to build a physical model and run it in a wind tunnel). So there isn't a "CFD let us try X" trend, there is "CFD let us try X, Y, Z, A B... and X ended up working."

I suppose I meant to ask, what are examples of X - the things that ended up working - in your analogy above. Sorry for the confusion!

Just wanted to follow up here @DJClayworth :)!


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What is the aerodynamic impact of the curved/crescenteric wing of the Boeing 787?
From certain angles (see pictures below), the wing of the Boeing 787 seems almost curved/crescenteric in shape, particularly closer to the wing-tip. While the Boeing 787 makes use of "raked wingtips", the curved wingtips seems to be slightly different to Boeing's original description of a raked wingtip in its patents, which seem to be solely reserved to discussing a true "rake" - a section of the wing that has a different sweep angle as the majority of the wing (but each part of the wing is still "straight" - so no curve).
What would the impact of a more "crescenteric" shape to the wing be from an aerodynamic perspective?
Note - I'm using the word "curved", but I do not intend to refer to the wing flex of the wing, which has been discussed in prior posts.
Edit: I've been asked whether this prior question already answers the question of the post. I do not think so, because the raked wingtip explanation refers only to a wingtip that has greater sweep angle than the base wing. But in this case, it seems to me that the 787 actually has a continuously varying sweep angle on the tip, giving rise to the "curve/crescent" shape seen in the image. This is an additional change in geometry/design that might have additional aerodynamic performance benefits.

`

I do not think so, because the raked wingtip refers only to a wingtip section that has a greater sweep angle than the main wing, whereas the 787 wingtip features a continuously varying sweep angle (creating a curved or crescent-like shape). But if Peter (the main commenter) wants to correct me, I'll close the question.

Boeing certified only one wingtip for the 787, so the other question applies. Your pictures make it look like a continuous curve but it really only has a generous radius in the transition from leading edge to rake along with a blended upsweep. The overhead drawing in the second answer makes the geometry more obvious.

Hm, I suppose I can close then.


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What increases the rotational velocity of air in an axial flow compressor?
In an axial flow compressor as used in modern jet engines, the goal of a compressor rotor is to increase the angular momentum of the air by imparting an increase in swirl velocity. (Note: the goal of an entire compressor stage, including rotor and stator, is to raise the pressure of air, but I'm focused purely on the rotor right now).
However, I'm a bit confused as to what imparts this increase in swirl velocity. Is it: a) the rotational motion of the rotors themselves? or b) the lift generating characteristics of the rotor airfoil shape.
Basically, I'm not sure why the blades of the rotor needs to be lift producing entities. It seems that if you had a flat plate it would also increase the rotational velocity to the flow because it itself is rotating. For example, in typical pictures of velocity triangles, nowhere is lift and or drag mentioned.
Hopefully that question makes sense!

"the goal of a compressor rotor is to increase the angular momentum of the air" -- is that the goal in & of itself, or something that happens on the way (i.e.  a means) to achieving another goal?  If you had lots of angular momentum but very slight axial velocity, would that accomplish any useful purpose?

The goal of the rotor is to add energy to the air stream. A rotating propeller/fan/turbine will always add some of this energy in the form of kinetic energy of swirling motion—to add energy the blades must do work, which means apply force on the air in the direction of their motion, and this will inevitably accelerate the air in that direction.
The blades do actually push the air aft, not just in the direction of rotation, but the swirling can't be avoided. So the stator—which can't add energy because, not moving, it can't do work—then redirects the air to convert the kinetic energy of the swirling motion to pressure (or kinetic energy of straight flow in case of a ducted fan; what form it will be depends on the pressure downstream).


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Specifying Climb rate in the Boeing 747-400 FMC
In the Aerowinx Precision Simulator PSX documentation for the FMC, I do not see how one can program a climb rate after takeoff.
I'm more interested however in the actual airplane. Suppose after clean-up, I want to maintain 500 FPM, is this something that I can only do via intervention with the V/S switch on the MCP, or is this programmatically achievable in the FMC?
Typically, at 400 AGL, VNAV engages with VNAV-SPD and THR-REF pitch and A/T modes, but the fixed climb thrust only allows the use of elevators to maintain the commanded pitch for airspeed. However, if I hit the V/S button and scroll to the target climbrate, then the A/T mode that automatically switches to SPD will control airspeed with thrust, and I can chase the horizontal FD bar to control climbrate with elevators. What I want to know is whether I can program this into the FMC, such that at the acceleration height or Flaps 5 setting, the engaged pitch mode automatically changes from VNAV-SPD to V/S. thanks.

VNAV has nothing to do with Climb Rate - if you want a VS use VS mode, else use VNAV or FLCH.
VNAV is trying to get the airplane as high as possible as soon as possible, so that it can cruise in the thin air for as long as possible.  "I want 500fpm" is incompatible with this, that's why you have the different vertical modes.
RAC

Thanks, but I’m well aware that VNAV does not control vertical speed, which is precisely why I was asking about whether the V/S functionality could be programmed into the FMC.

In fairness, I can see how the 2nd paragraph of my question might have been a bit confusing, so I’ve rephrased it to clarify my precise intent.

@SharatVChandrasekhar but still, why would you want to do that? For climb it is both more efficient, and safer, to set climb power and maintain speed with pitch, that is the FLCH mode.

It's not about me specifically wanting to do it - and frankly I wouldn't want to. My question is quite simply - out of academic interest - whether this can be programmed into the FMC. By the way, there are some operating procedures where at MGTOW, a climb rate of 500 FPM is suggested up to the clean maneuvering speed of 250 to 260 KIAS (VREF +100). See page 10 of the PDF at this link: https://kingla.files.wordpress.com/2011/07/boeing747-400flightmanual.pdf.

As you will note, they don't suggest using V/S either, but setting climb power and accepting whatever they get between 500 -- 1000 FPM.  I'm just curious as to whether there is any feature in the FMC where it accepts a climbrate specification.

The FMC is about planning the flight, and you wouldn't normally plan to use VS - that's generally used to rescue a plan that's not working.

Understood. Thanks for the clarification. A quick follow-up: Does FL CH eliminate all altitude constraints if  a higher altitude is selected in the Altitude window?


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Why require a minimum of 1500 hours to become an airline pilot?
In the United States, under part 61 standards for pilot certifications, for a person to apply for an airline transport pilot certificate one of the aeronautical experience minimums is a minimum of 1500 hours total flight time. Most ICAO compliant countries require similar numbers.  My first question on this would be why so many? It’s true that you want airline pilots to be experienced and no doubt I would not expect an airline to make someone a captain of a large, turbine airplane without a minimum of say 2000 hours with a lot of that being hard instrument and weather time but why require a first officer to need a minimum of 1500 hours for an ATP?
I’m aware prior to 2014 that the FAA allowed part 121 first officers to fly with only a commercial pilot’s license for the right seat and the 1500 hour rule only came about after the Colgan air crash in 2009.  People argue that this is what is resulted in only one commercial airline fatality over the past 10 years.  Still I wonder why an arbitrary number 1500 was chosen for an airline transport pilot certificate. Now I am also aware that this is not always the case. For instance graduates of approved part 141 schools can get a ATP with only 1200 hours TT with a aviation related associates degree, 1000 hours TT with an aviation related bachelor’s degree and military pilots with 750 hours TT can apply for an ATP directly.  My only thoughts would be, if part 61 training is so terrible that it requires a person to put in 1500 hours in a Cessna prior to applying for an ATP, what makes an aviation bachelors program superior or military flight training as compared to it, and why not simply train pilots under parts 61 to the same standards as the military does?
EDIT
Some other thoughts on this topic:  While the military does allow people to operate large turbine powered aircraft with FAR less that 1500 TT, they’re in a position where they simply have no other alternative.  You can’t give a fresh 2nd Lieutenant 1500 hours in a T-6 or T-38 prior to dispatching him/her to an F-16 squadron somewhere as it’s simply not affordable.  Even these trainer aircraft cost several thousand an hour to operate and training budgets are already tight.  So they’re sent off to a active unit with 200 to 300 hours total time.  The downside is that, even in peacetime, military aviation units suffer horrendously high accident rates as opposed to their Part 121 counterparts.  As I understand it, the USAF alone had 39 class 1 mishaps in 2018.  It’s an unfortunate fact that, as much as primary flight training could teach you, it’s only a license to practice your craft; flying is an artform and takes a lifetime to master and there are a lot of dangers out there.  That being said, mass hours alone are really not good experience.  It requires APPLICABLE experience to matter.  An aviator trained in a small part 61 flight school in central Iowa and logs 1500 hours flying Cessnas locally as a CFI is nowhere near ready for the rigors and stress of an airline pilot, but a pilot with 500-600 hours TT with complex, instrument, and multi-engine turbine time in high density airspace around major cities is far more likely to success here.
It bothers me that these kinds of regulations are written this way, without any thought to the practicality of it. It’s also strange that way, since, most of the FARs are very practical and based on well researched information as well as accidents in the past.  It also bothers me that it causes new pilots to bear an extreme burden to build those kinds of flight hours for a career, which may or may not be beneficial to them. I’m just curious what the FAA’s thought process was in recommending the scheme

I have military flight training but I still needed 1500 hours for an ATP.  And while I agree my military training was high quality, I would disparage the quality of some of the 141 and 61 training I have been exposed to.  (What do you mean by military “standards”?). That said, the short answer is that you have to draw the line somewhere, and any debate on its location will be heavily opinion based.  This reads like a lament and I don’t think there is a factual answer so I’m voting to close.

... although it does make for interesting discussion.

Just a comment on the military peacetime mishap rate:  Even in peacetime pilots train for war.  If you are implying the rate has to do with inexperience I would submit it it has as much or more to do with the inherent hazards of the mission.  If military pilots simply ferried aircraft the numbers would be far different, so you really can't draw a fair comparison with their civilian counterparts.

Colgan was a smokescreen allowing the administration to cave in to pressure from the ALPA union to restrict the number of pilots available so they could demand higher wages.  FO hours required went from 250 to 1500. However, the Colgan pilot had 3379 flight hours and the FO had 2244, so lack of experience had nothing to do with this crash. All the "fix" did was break the pilot training process in the US.

This doesn't read like a question, more like random musings.

@Pilothead Why didn't anybody bring this up at the time?

@Michael This was well known but objections fell on the deaf ears of Obama and the Democrat controlled House and Senate. It was a payoff to a union whose members already made more than the median doctor. No other country in the world has followed suit; Canada requires 250 hours (unchanged) and Europe 230. The US is no longer competitive in pilot training. If there were a shred of sanity in the rule someone else would have adopted it in the fourteen years since instituted. Forbes wrote an article "the 1500 hour rule has broken the pilot pipeline in the US" which explains.

If I understand correctly, your main focus is the history of 61.159 and the literal question in your title: why does the FAA require 1500hrs for an ATP?
The short answer is that the FAA was complying with ICAO's ATP requirements. This is from the Federal Register in 2009:

International Compatibility
In keeping with U.S. obligations under the Convention on International
Civil Aviation, it is FAA policy to comply with International Civil
Aviation Organization (ICAO) Standards and Recommended Practices to
the maximum extent practicable. There is one revision in this final
rule document (See Revision No. 71) where the FAA has amended
§ 61.159(d) and (e) to conform our ATP certification requirements to
ICAO Standards and Recommended Practices.

Here's the specific 1500hr requirement:

The current FAA regulation applies an obsolete ICAO ATP airplane
aeronautical experience rule. Before 1974, ICAO only required 1,200
hours of total flight time to qualify for an ATP certificate in the
airplane category. In 1974, ICAO amended its ATP aeronautical
experience requirements for the airplane category to require 1,500
hours of flight time as a pilot [...] This revised change harmonizes FAA regulations to ICAO's current standard.

There were a few other changes for ICAO compliance in the same rule. And before you ask, I have no idea why ICAO chose 1500hrs :-)
You also asked why a first officer requires 1500hrs. As you mentioned, that was in response to the Colgan Air Flight 3407 crash but it was a political decision by Congress and didn't originate with the FAA. From the Federal Register again:

The Airline Safety and Federal Aviation Administration Extension Act
of 2010 (Pub. L. 111-216) directed the FAA to conduct a rulemaking to
improve the qualifications and training for pilots serving in air
carrier operations. Specifically, section 216 of the Act focused on
the qualifications of air carrier pilots and directed the FAA to issue
a rule that would require all pilots serving in part 121 air carrier
operations to hold an ATP certificate by August 2, 2013.
[...]
Section 217 also directed the FAA to ensure pilots have sufficient
flight hours in difficult operational conditions that may be
encountered in air carrier operations and stated that the minimum
total flight hours to be qualified for an ATP certificate shall be at
least 1,500 flight hours.

The wording of section 217 of the Act includes:

(1) Numbers of flight hours.--The total flight hours
required by the Administrator under subsection (b)(1) shall be
at least 1,500 flight hours.

That change seems to be largely due to lobbying by the families of the crash victims, who include it on their website as a "legislative accomplishment". I couldn't easily find any more detailed sources about why they made that specific point a goal.
I realize that this doesn't explain why ICAO wanted 1500hrs, or why the FAA previously wanted 1200hrs. The Federal Register online only goes back to 1994 and it would likely take some more 'serious' research to find out more about their original reasoning.

Correct me if I'm wrong, but didn't both Colgan pilots have in excess of 1500 hours?

@RetiredATC yep, it's a knee jerk law without any foundation in reality


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Is there any API for planes with ICAO identifiers?
I'm looking for an API that I can use for images and details of all planes, preferably searched by the name of the plane or the ICAO identifier of the plane.
Right now, I've found a list on Wikipedia with every single aircraft ICAO identifier.
Furthermore, I've found a website with most planes by ICAO, but some things, for example the (S65C) Eurocopter AS365 Dauphin aren't included. That's the thing, it's not meant to be an API, just a collection of images.
Even with those, I can't run an API search for the name and stats for the plane.
Anything can work for me, if I have to, I can use multiple services, but all I really care for are:

Plane stats
Aircraft name
ICAO identifier
Image of plane

And those all have to be searched for by the ICAO and the aircraft model.
Also, I don't want completely paid APIs. I'm fine with keys that have a limited amount of calls in a certain period of time, but nothing that you have to pay for.
NOTE: I'm not looking for ICAO airport identifiers, that's an entirely different thing.

ICAO DOC8643 API is available here: https://www.icao.int/Aviation-API-Data-Service/Pages/default.aspx

Adding the @60levelchange link info, the structure of the queries and the responses.

@60levelchange Oh, yes, nice resource, but I forgot to mention that I don't want paid APIs. I will edit my post.

Also, @60levelchange, do you know how to get an ICAO api key? I saw very expensive (talking about like $20K) api calls, but I didn't find out how to get a key.

Once again, @60levelchange, I searched up "doc8643" which is a document (that you can buy) that has all the identifiers. I came across this free doc8643 website here, but it doesn't seem like I can access it in any way, other than web scraping which I have no idea how to do.

You're not going to find DOC8643 for free anywhere (at least not an up-to-date version). ICAO charges big bucks for their documents, unfortunately

@60levelchange LOL, that's so ironic, because I just did. While searching on NPM, I found this package. (To view it, click on "resources")

Oh, I guess it's not up to date.

Just to clarify, you are looking for aircraft types rather than the planes themselves, right? Also exactly what details are you looking for?

I'm trying to make an API that searches for the plane using ICAO identifier and then returns images, stats, and most importantly aircraft name

Not sure what's "ironic" about it. That link is to a 4-year old version of DOC8643. As I said, you very likely won't find an up-to-date version for free.


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Why don't planes (mostly airliners) primarily use GPS for navigation?
For the four airliners that I use in Microsoft Flight Simulator (A320neo, A319, A310-300, and 747-8I), I always have to align IRS and set up RAD NAV on the MCDU. Why can't we just use GPS?
On the 747, for setting up the IRS, I literally just have to set the knobs to NAV and copy the GPS position into the IRS field on the CDU. This is nonsense. Why do you need three different navigation methods when you can get one GPS?

So, you go the extra mile to design a redundant avionic system based on two different design and manufacturing teams, with different microprocessors and associated circuits, different computer architectures and different functional specifications and then you feed it with one single unreliable government's owned input signal? That doesn't make much sense

@sophit AFAIK the 3 ADIRSs are all identical on an Airbus. The "two different design and manufacturing teams, with different ..." apply to the flight control computers (ELAC, SEC and FAC), which are not involved in navigation.

@sophit uhm, 4 different governments (counting the EU as one) actually. Because when we say GPS most people just mean satellite-based navigation, of which GPS was only the first (hence the name stuck), but most receivers are fully capable of switching between the data sources and some always compute several at once to spot oddities

@Hobbamok none of the aviation grade GNSS receivers that I know do any of the things you describe.

https://www.ntsb.gov/investigations/AccidentReports/Reports/mar9701.pdf is worth a read.

How do you detect wind shears or yaw alterations (to prevent Dutch roll) using a GNSS receiver with a refresh rate of 10 Hz? INS uses are not limited to navigation. Likely a duplicate of Do today's aircraft still have INS (inertial navigation system)?

The question title and question description don't seem to quite match up. It seems that your question should be titled, "Why don't planes (mostly airliners) only use GPS for navigation?". This would better match up with your other statements, "Why can't we just use GPS?" and "Why do you need three different navigation methods when you can get one GPS?"  I think this would be a more clear title because "Why don't planes (mostly airliners) primarily use GPS for navigation?" is really a different question even though many of the answers might be similar.

Satellite-based navigation is not reliable.  The operator can turn it off.  The satellites can fail.  The signal can be jammed or spoofed.

I saw the GPS procedures at a major airport all out of service because someone bought a cheap piece of electronics which interfered with the GPS signal in the area so much that the procedures were unusable. It was unintentional and the device coat about $50. It takes some time to find the device once this occurs. Meanwhile all aircraft flying into and out of this airport were using non-GPS procedures.

More recently, in 2022 Russia jammed GPS along the Finnish border, reaching into Finland. One airport, Savonlinna, only has GPS approaches and had to shut down. Some airlines operating small planes with only GPS navigation had to cancel flights in the area. https://www.flyingmag.com/pilots-in-eastern-finland-warned-of-gps-interference-near-russian-border/

All of that is possible with standard radio navigation.  It’s even easier to block aircraft navigation aids as it’s as a lower frequency and can easily be matched by cheat transmitters.  Note, it is highly illegal to interfere with licenses radio frequencies especially those used to navigation, public safety and communications. https://www.fcc.gov/general/jammer-enforcement

And the driver noted above was fired and fined over $30k.  And just as easily could have caused more issues.  Might also note that when GPS jammers were used against the US military they were very useless.  They can work over a localized area but have no practical effect over large territories.

Do military aircraft largely use satellite-based navigation?

How are all of your statements not true of all other forms of electronic navigation?

@MatthewWhited That's mainly because the military does not care too much if one drone out of twenty gets lost and crashes, as long as the others hit their targets (which is exactly what happens with localized jamming). Armed conflict of any sort is incredibly wasteful anyway. However, civil aviation has a very different level of risk tolerance.

@Sam To mess up your inertial navigation, somebody has to get inside your plane.  The overall point here is that you should not rely on only one source of navigation information.  Redundancy!

In fact GPSS signals are disabled for testing frequently, not the whole system but targeted bubbles with a radius of about 400miles. (horizontal radius at ground is much smaller than the horizontal radius at commercial cruise altitudes). These zones are in NOTAMS that affect my area of the USA about once per month.

Note that jamming is the polite attack against GPS. The attacked party at least knows something is wrong. There are also methods that can hijack the signal to make the receiver think it's several miles off from their true location, without suspecting anything is wrong. There are demos against phones, though harder I'm sure the same is possible against aircraft receivers that rely solely on GPS.

@Sam theoretically there is no difference, but practically there is a world of difference. Terrestrial navigation methods use significantly stronger signals (because the transmitter is not 20,000km away) and the protocols are significantly simpler making them harder to hijack. They also use many different frequencies, often with readily available backups. Practically, there is a long list of real-world attacks against GPS, can you list attacks against terrestrial navigation beacons? This also answers the OP's question: It's much easier to hijack only GPS than GPS and the terrestrial backups.

@Adam Not true. In this incident, an airplane was taking off, properly spaced, and it reflected the ILS signal so that a 777 swerved off the runway. Another example is why you can't use FM radio receivers on a plane, the local oscillators are known to interfere with receivers. In this incident, a fault in a single ILS transmitter nearly killed a planeload of people.

@Adam In this incident, a malfunctioning VOR sent a plane to within 55 feet of a hill. GPS with RAIM could easily detect a single malfunctioning satellite, all of these near-craters all relied on a single source.

@user71659 those are great examples for how relying on just one technology can get one in trouble. But neither example is an attack. The ILS incident is from an obstruction in front of the antenna (another aircraft taking off), not an antenna fault. There was another incident of an aircraft parked too close to the runway and their tailfin causing radio reflections with a similar effect. They only way to turn that into an attack (intentionally causing an accident) is to have magnificent timing like in your example or intentionally reject tower orders and have them not notice.

Also ILS is a bad example because it does not have an alternative. GPS can guide you to the runway, but it's not accurate enough to land you or guide your rollout like ILS can. Military GPS might be, but they have higher accuracy. They use a second frequency to measure (and cancel out) some ionosphere variability. Civilian GPS cannot use this second frequency, and official civilian GPS uncertainty is higher than many runway widths. Yes there are tricks to do better most of the time, but good luck certifying that for use as an only aid.

@Adam "GPS can guide you to the runway, but it's not accurate enough to land you or guide your rollout like ILS can". Not true, GLS (GNSS Landing System, formerly LAAS) has live in airports for a decade now. The fact is GPS has redundancy with multiple satellites, the cost of installing and maintaining ground-based nav systems mean redundancy doesn't happen in practice.

@user71659 That's for a CAT1 approach. Your linked accident was a CAT3. Airbus says, "Nevertheless, VHF-radio based ILS remains, for the time being, the only certified means for a commercial airliner to perform a CAT2 approach or a CAT3B ‘auto-land’ onto the runway and subsequent rollout." https://aircraft.airbus.com/en/newsroom/news/2022-06-satellite-based-landing-system

@Adam Again, incorrect on multiple points. CAT II GLS is currently operational. If you actually read the swerving incident, they were performing an approach in CAT I conditions, that's why the ILS wasn't protected.

While the other answers are correct about why GPS (or similar GNSS, such as GLONASS, etc.) aren't used exclusively for aircraft navigation, I would say that, these days, the premise of this question is no longer correct and GPS is the primary means of navigation for aircraft. It just is not the exclusive one and it most likely never will be because the aviation industry very understandably likes having backups to their backups to their backups, ideally ones that do not have common modes of failure, as would be the case with using other GNSSs.
That said, due to the extreme cost of getting new hardware certified and the slow nature of regulatory changes, the aviation industry has a lot of momentum and major changes take a long time, both to be approved for initial use and then to actually become ubiquitous. Airplanes, once built, tend to have very long service lives, especially when talking about an entire type of airplane rather than just one particular airplane. As an example, I'm in my late 30s and every airplane I've flown was built before I was born and was based on a type designed in the 1950s and/or 1960s. While airliners don't usually stay in service that long, we're still talking about production runs that last for decades and then service lives of decades beyond that.
Of the 4 aircraft that you mentioned, the second-newest of those types is the 747-8. Aside from being based on an airframe originally designed in the 1960s and designed to be flown by pilots who had been trained on previous versions of the 747, even the 747-8 itself began design in the early 2000s. The A310 was designed before GPS was even allowed to be used for civilian purposes and the A320 family (of which the A319 is a part) was also already well into its design phase before GPS was opened up to civilian aviation, though it had become available before it first flew. At the time the A310 and A320 were designed, airliners were still sometimes navigated at night in part by looking at stars with a sextant!
As for inertial reference systems specifically and why they are used in addition to GPS, they are actually great and can give much faster and more precise measurements than GPS in the short term. Orders of magnitude more precise. Additionally, they provide orientation information that GPS does not (i.e. GPS will not tell you if you are rolled to 35 degrees right bank angle and pitched 15 degrees nose-up, whereas the IRS will.) Over the course of a whole flight, though, their drift can become significant without something like GPS or radionavigation to give them occasional fixes. This is actually how your smart phone (or other portable electronic device) works, too. It uses GPS (and cell towers and other such things) for occasional fixes to factor out drift, but it uses an IRS with accelerometers and such that measure acceleration in different axes in order to work out short-term position and orientation changes. The same is true for virtual/augmented reality systems and such, except that they use optical systems for their fixes rather than GPS or radio signals (which wouldn't be nearly precise enough for their purposes.)

For many purposes the GPS is good is enough.  Systems certified for RNP after all provide a 1x10-7 per flight hour containment guarantee and decent availability numbers. For other purposes, like total loss of position in IFR conditions, that may not meet the safety numbers wanted by engineers and airlines.
There are numerous reasons why IRU's are still installed, so often GPS provides the en-route navigation and perhaps the approach, while IRU provides a backup or cross-check for Position Alert messages. Of course you'll still need ILS, VOR, etc. for some approaches for mostly historical reasons that are too complex to get into here.
The biggest issue with relying solely on GPS is jamming. According to reporting by IEEE Spectrum lost or intermittent GPS was reported to the FAA 233 times in eight months in 2017-2018. That's alarming if you thought GPS was rock-solid reliable. Compared against millions of flights per year, it's nothing to lose sleep over but much worse than a good IRU since IRU have virtually no external interference. GPS issues may not be common at your home airport, but there are geographic areas where GPS issues are pretty common like Tel Aviv Airport, Ukraine (even before the war), the Turkey-Iraq border, and even White Sands Missile Range in New Mexico. (source: ASRS reports and discussions with pilots).

I literally just have to set the knobs to NAV and copy the GPS position into the IRS field on the CDU.

The user interface here is clunky, but the point is that you should only copy the GPS position if you trust it.  It's like manually entering baro correction on both sides of the cockpit, or setting the heading on a directional gyro- if automated too much then a single fault could have catastrophic consequences. Could you design a unit that automatically syncs the GPS position repeatedly to the IRS as long as they don't start diverging?  Yes, and that's more or less how a GPAHRS/GPIRU works.
Is GPS widely used, and increasing in usage? Yes. Coupled GPS-AHRS are replacing one or all of the IRUs in many new airplanes, so many commercial aircraft out there like CRJ 700's won't dispatch if all their GPS units experience issues. Overlay approaches are replacing VOR/DME, and there are now more LPV approaches than CAT I ILS approaches (source). The ADS-B mandate probably never would have happened without widespread availability of certified GPS units. However GPS's not the only source of position in any cockpit, just like you don't rely solely on one pitot tube, and like how magnetic compasses struggle in areas of magnetic unreliability like Manitou Springs.

"if you thought GPS was rock-solid reliable", The 2-sigma interval of the GPS specs (URE < 2m), indicates the value is off-specification 5% of the time. This is not well known, even if this represents 36 h per month.

@mins In practice the avionics try to abstract the URE < 2m into ANP/EPE, and then further into a worst-case analysis through RAIM or SBAS that provide a 1E-7 Horizontal Protection Limit. I agree with you that the takeaway for pilots is you are frequently going to be off your indicated position by more than ANP/EPE.

This doesn't seem to be explicitly stated in any other answer: a significant part of procedures for flying airplanes comes from a desire for reliability, redundancy and fail-safes.
I can't tell from your question whether you are a simulator-only pilot or whether you fly physical airplanes. I can relate from my own experience, many hours of an instructor shutting off or covering up a random piece of equipment in my cockpit.
I enjoy simulators as well, and sometimes when things aren't going my way I just reset the simulation. This is not an option at 1500 feet going 80 knots. It was scary enough with advance notice of precisely what would fail and an instructor sitting next to me.
Luckily, I've never been in the position of having a real problem of this sort. I can just imagine what it would be like though, with 300 people behind me and approaching an airport, to find out my GPS doesn't work. Some other system may not work, but I only need one of them to work to land the plane.

IRS are used for attitude reference, not just location. GPSS has no way to know if you are flying upside down and sidewards, it can only show where your antenna was located when the last set of signals was received, not its angle or acceleration.
GPSS signals are disabled for testing frequently, not the whole system but targeted bubbles with a radius of about 400miles. (horizontal radius at ground is much smaller than the horizontal radius at commercial cruise altitudes). These zones are in NOTAMS that affect my area of the USA about once per month.
In many mountain valley airports, there are not just enough satellites in view regardless of testing or outages.
Civilian GPSS alone does not meet the accuracy and reliability standards for precision approaches, and the position is always slightly behind (shows where you were, not where you are) due the nature of receiving signals and processing them. The IRS is much more accurate over short times, distances, and rapid changes in attitude and speed. Accuracy in this case is not judged on normal, best-case, or average performance it is judged based on expected worst-case performance(ie bottom 1% or 0.001%). Vertical reliability and accuracy seems to be the most limiting factor, though I can't explain the full engineering reason, approaches have the least buffer in the vertical dimension and GPSS is about half as accurate in the vertical.
If augmented with a local (at the destination airport) ground-based signal correction system(LAAS/GBAS) it can qualify as "precision" however the IRS or other combination of instruments and NAV aids are still needed to deal with a sudden failure of any part of that augmented GPSS, multiple external signals or internal processing. Only a few airports have installed local augmentation stations at this time. Though somewhat less accurate, wide area augmentation(SBAS) is a suitable alternative for en-route and less critical approaches. Upgrading a small plane from traditional ground transmission navigation(NDB VOR ILS etc) to a basic WAAS capable GPSS nav system is about \$10000usd, one with a big screen and many features is \$50000usd, which is close to the cost of purchasing a complete airworthy old small airplane.
Military GPSS has an additional inherent correction signal from the standard constellation to deal with fluctuations in the ionosphere. But even the one-time use military GPSS equipment (eg cruise missiles) are backed up by internal IRS. To deal with both signal jamming and rapid maneuvers.

"the position is always slightly behind", while true especially for older 1 hz receivers, but in practice this latency is relatively low. Some regulations recommend low end-to-end latency.  AC 20-165 for ADS-B recommends total measurement-to-transmission latency of less than 2 seconds, and "uncompensated latency" (e.g. after extrapolation) of less than 0.6 seconds. You bring up lots of other good points like that regulations sometimes require backup in case of loss of GNSS.


text:
What kind of feedback is this?
i found this link that shows a united airlines intercom feeding back and i was just wondering how typical is this on these flights?? i’ve never heard feedback come back frequently every time the speakers are on
is it from the mic?

i need an answer

I don't think you will get any verifiably correct answers because this isn't a reportable incident that the FAA tracks.  FWIW, I have never noticed feedback being a problem.  And why do you "need" an answer, vs just want to know?  P.S.  This is probably a better fit on Travel SE.

This is standard audio feedback caused in the standard way: The active microphone was too close to a speaker, generating a feedback loop.
We can't say for certain why this happened here but not on most flights. Perhaps the flight attendant was closer to a speaker than usual.


text:
How does camber affect on critical aoa?
i am pretty clear about why a higher camber would increase Clmax, but critical aoa.
one of the resources i read the graphic shows a greater chamber near the leading edge will increase Cl at different aoa and Clmax, a greater that of trailing edge will increase both Clmax and crtical aoa.(i can't post a link due to the web, i'm in china, really sorry for that)
as far as i know camber would increase the gradient which leads to a wilder separation, that's going to make the airfoil easier stall.
but on the other hand, may be a greater camber would increase the performance of aircrafts at larger aoa? like a leading edge flap?
i have went through a lot of conversations about camber-Cl curve relations, they are basically about Clmax.
many thanks

In the sub-sonic realm, where most people fly, we try to avoid flow separation, which essentially is a breakdown of the low pressure area above the wing.
Starting with a flat plate, as we raise Angle of Attack, at a given velocity, for a given chord (Reynolds number), a stable area of low pressure will form over the wing.  Lift will increase as we raise AoA, until the airflow gets disrupted enough to collapse the bubble.  This disrupted airflow reduces lift and greatly increases drag, and is known as "stall".
Increasing the camber from a flat plate keeps the airflow "smooth" at higher angles of attack, giving much greater available Coefficient of Lift.
The Davis wing is a good example of a sub-sonic airfoil optimized for maximum Clift.
Choosing degree of camber is dependent on chord and airspeed to get optimal flow (best lift to drag ratio) for a given airspeed.  There is a wealth of knowledge from 120 years of aviation experience to be had at www.airfoiltools.com.
Generally, increasing camber increases stall AoA.

@Raketenolli I suggest you go to airfoil tools.  0012, 2412, and 4412 indeed showed that increasing camber increases stall AoA, not surprisingly more so at lower Reynolds numbers.  So, please take a time out, and maybe come up with a decent answer to contribute to this thread.  (Reynolds over 10 × 10$^6$ may be interesting, but hey, that might be why airliners know when to fold 'em).

those tools are amazing, thanks a lot.


text:
How to calculate sink rate from wing measurements
First of all, sorry to bother you when I am coming from a bird rather than aircraft background.
I have two questions that I'm struggling to get an answer for.
Firstly, how is sink rate calculated (estimated) given mass, wing area and wingspan? Assume air speed/density and the other variables are known.
I know others use these variables to calculate sink rate but I can't find how. I'd like to know the formula so I can try to calculate sink rate myself.
Secondly (though this should hopefully become more clear once I learn the answer to the first question), is sink rate affected by aspect ratio? I am assume it has a lot to do with wing loading, but does aspect ratio reduce it much?

Sink rate is greatly affected by control inputs, i.e. the airspeed being flown. In its present state, too little information is given to answer the question.

Hi @RalphJ, thanks. I was hoping to get the formula so those variables could be factored in. I'll try to make the question a bit clearer.

Additionally, ask your 2nd question as a 2nd question once you get the answer to this one. That will allow people to focus on answering one question at a time and prevent a conundrum of how you choose which is the accepted answer when you have one good answer for the first question and another good answer for the second question. Plus, it's the rules, as noted in the [tour] and [help].

@FreeMan, while often questions are different enough justify separating them, in this case they are so similar I don't see the point.  Ultimately the best answer will include consideration of ALL aerodynamic forces on the airframe.  It would serve no useful purpose to ask a fresh question for each individual contributing variable such as camber, drag, wing area, mass, etc.  JMHO.

(I not sure if new ASE contributors have chat capability but this would be best explored further in chat.)

Aeroplane.
Force equilibrium,:$$ -D - W \cdot sin \text{ } \gamma = 0 \tag{1}$$ $$+L - W \cdot cos \text{ }  \gamma = 0 \tag{2}$$
With $\bar{\gamma}$ = -$\gamma$ being the positive glide angle, and with the lift and drag equations (with S = wing area): $$C_D \cdot \frac{1}{2} \rho V^2 \cdot S = W \cdot sin \text{ } \bar{\gamma} \tag{3}$$ $$C_L \cdot \frac{1}{2} \rho V^2 \cdot S = W \cdot cos \text{ } \bar{\gamma} \tag{4}$$
Divide equations (3) and (4): $$tan \text{ } \bar{\gamma} = C_D/C_L \tag{5}$$
In a glide, the glide angle is often small, and $cos \text{ } \bar{\gamma}$ can be approximated as 1. So now for airspeed and sink speed $\bar{C}$ follows: $$V ≈ \sqrt{\frac{W}{S} \cdot \frac{2}{\rho} \cdot \frac{1}{C_L}} \tag{6}$$ $$\bar{C} = V\cdot sin \text{ } \bar{\gamma} = V \cdot \frac{C_D}{C_L} \cdot cos \text{ } \bar{\gamma} ≈ \sqrt{\frac{W}{S} \cdot \frac{2}{\rho} \cdot \frac{{C_D}^2}{{C_L}^3}} \tag{7}$$
$C_L$ and $C_D$ as function of angle of attack $\alpha$ need to be known, in order to find the maximum value of ${C_L}^3/{C_D}^2$ equating to the minimum sink rate. For the Reynolds numbers that flight takes place in, and for low subsonic speeds, $C_L$ follows from (4), and $C_D$ is approximated as $$C_D = C_{D0} + \frac{{C_L}^2}{\pi A e} \tag{8}$$ with A = aspect ratio = $b^2/S$ with $b$ = wing span. $C_{D0}$ needs to be established in a wing tunnel, or with CFD, or by referring to comparable craft in design books. As for the Oswald factor $e$.

Birds.
Equation {7} remains valid for birds, but how much are the $C_L$ and $C_D$ values for birds in flight? There is a reference in Siegfried Hörner's book Fluid Dynamic Drag, page 7-22:

A buzzard was ingeniously tested in free flight...by tracking (following) him in a sailplane. The L/D...ratio of that bird was determined using the calibrated characteristics of the plane as a measure.

The measurements found a max buzzard $C_L$ of 1.3 at the lowest speed of 8.5 m/s, and a $C_{D0}$ of 0.009 with reference to wing area S. The pic above shows the buzzard data under (a) in the left lower corner. The sea gull data was measured on a plaster model in a wind tunnel, and Hörner places question marks on the drag numbers found, probably due to the feathers not being properly reproduced.
If the bird sink rate needs to be computed from mass, wing area and wing span only, equation (8) will need to be re-worked to fit the measured data. The measured data line follows the $C_D$ vs. ${C_L}^2$ trend approximately.

The mechanism for vultures/eagles may be better in thermals "raptors typically fan tails and sweep wings slightly forward, while opening primary feathers as wide as possible".  Not the lowest drag for gliding for distance in still air, but best for "sailing" the updraft.  Wing tip feathers may also generate thrust from the updraft.

@RobertDiGiovanni Hörner mentions that they measured the buzzard while gliding and while soaring, during the latter the wing tip feathers were extended.

Maximizing wing (+tail) area helps to minimize the sink rate, but not to maximize the still-air glide ratio. All things being equal, if wing area were fixed, increasing the aspect ratio should normally decrease the sink rate, but it makes sense that in the real world birds minimize sink rate by maximizing wing (and tail) area, even if aspect ratio is slightly decreased.
One formula for aspect ratio is (wingspan squared) / area. Birds generally have lifting tails, at least when broadly fanned, so it would seem reasonable to include the tail area in the calculation. I'm sure you've observed how raptors typically fan tails and sweep wings slightly forward, while opening the primary feathers as wide as possible, to minimize sink rate (maximize climb rate) in a thermal updraft. This configuration maximizes lift coefficient and also wing (and tail) area, but decreases aspect ratio somewhat.
For fixed wing area (which is not accurate for birds), min sink rate is achieved at max value of (lift coefficient cubed) / (drag coefficient squared), while flattest glide angle in still air is achieved at max ratio of lift coefficient / drag coefficient. Basically this means that minimizing drag is more important when you are trying to maximize still-air glide angle than when you are trying to minimize sink rate.
For a given wing area, increasing the aspect ratio will generally minimize the sink rate, but at the low Reynolds numbers experienced by smaller birds this is not always true.

The question involves Area and Aspect ratio.
From the Lift Equation we have:
Weight = Lift = Density × Area × Velocity$^2$ x Coefficient
Coefficient is Angle of Attack of a given airfoil.  This data can be found at airfoiltools.
If you know your weight, design airspeed, fluid medium density (generally of air) and Coefficient at optimal angle of attack, Area can be calculated.
Sink rate (for a glider) will determined by the amount of drag produced at a given velocity to make Lift.  Drag is equal to the amount of forward force (provided by gravity) to keep your velocity constant.  This force is readily obtained from the glide ratio × the aircraft weight or can be deduced from the Lift/Drag ratio of the airfoil plus the form drag of the aircraft.
So now you test your sink rate at various airspeeds to build your sink rate vs velocity curve.  We can see induced drag will be greater at lower speeds, and form drag is higher at greater speeds.  The lowest drag value for the sum of the two is Vbg.
Too small a wing means too much speed is needed, too large is simply adding extra weight and drag.
If you get this far with a good strong rectangular Aspect Ratio 8 wing, glide ratio can be improved by increasing AR to 10 or more, at the expense of strength.  This is more fine tuning.  Gliding birds, such as albatross, have high AR, whereas acrobatic birds, such as hawks, favor lower AR strength.


text:
What does “ADS-B IDENT” mean？
What do pilots do when ATC tell them "ADS-B IDENT" (for an Airbus A-330)?
It happened when the aircraft flew into Singapore FIR at FL350. At the first contact with ATC, they are told "ADS-B IDENT".
Is there any description in some manual about this term?

As per the ICAO ADS-B Implementation and Operations Guidance Document:

5.8.1 Phraseology Standard
[...] TO REQUEST THE OPERATION OF THE ADS-B IDENT FEATURE
TRANSMIT ADS-B IDENT.

As others have mentioned, similar to asking an aircraft to "SQUAWK IDENT", but outside conventional radar range.

Welcome to Av.SE! Please link to the quoted document so others who are interested can read it. Thanks!

I am not sure exactly. We would definitely need more context to answer your question. Namely, where did you hear this (what country and what type of controller) and when (was the aircraft establishing initial contact with ATC, departing a towered or a non-towered airport, approaching land after flying in oceanic airspace, etc).
Without the term "ADS-B" attached to the front, the instruction IDENT means that the pilot should activate the "IDENT" function of their transponder.
See this question: How does IDENT work? for details on why ATC would tell a pilot to do this and the mechanics of how this works.
Since ADS-B broadcasts are very similar to secondary radar replies, the IDENT feature works even when an aircraft is in an ADS-B-only environment (not within secondary radar coverage).
The term "ADS-B IDENT" may also refer to the flight identification which is continually broadcast by the ADS-B unit on the aircraft; this identification may be a registration number (like N123AB or G-ABCD) or airline identification and flight number (like HAL41 or BAW123A). The controller may have been telling the pilot that the identification text being broadcast did not match the actual identification of the flight. This question provides some more information.

A colleague asked me about that question,so I'm not sure about the detail.
It's about 6 years ago,mabe more.
They flew into Singapore FIR.When first contact with ATC,they heared"ADS-B IDENT"

It is difficult to surmise without more context, but rather than a command telling them to activate the IDENT button, perhaps ATC was informing them that they had already identified them based on their ADS-B out signal.  Maybe they didn't have radar contact, but were letting them know they were receiving enough information to positively identify them so that they wouldn't need to IDENT.
Like when ATC calls traffic for a flight, and rather than visual contact the pilot responds that they are showing the traffic on their ADS-B display.

This is a decent possibility as well. While the FAA phraseology is "RADAR CONTACT" the ICAO phraseology is "IDENTIFIED" which (as pointed out by this answer) may be a more strictly correct term if ADS-B was used for the identification instead of PSR/SSR. I don't believe adding the specific method used ("ADS-B IDENTIFIED") is an ICAO requirement; it would be interesting to look at the Singaporean ATC handbook.


text:
Can flaps on Airbus/Boeing airliners be deployed at high speeds during cruise, still?
Brief Background of the Incident
Mayday Season 22 Features an Episode "Terror over Michigan" (Episode 6) based on the events of TWA Flight 841. The Episode revolves around a Boeing 727 cruising at 39000ft, Suddenly Loses 34000ft altitude in about a minute.
The episode depicts a shortcut (deviation from the SOPs) which involves pulling a circuit breaker leading to prevention of automatic extension of slats when the flaps are deployed (Flaps 9, I guess). The investigation talks about this procedure used sometimes to improve the performance of the aircraft while in cruise (which involves extension of flaps but NOT slats)
The episode takes a course where the circuit breaker pulled was accidentally pushed back again which caused the unintentional extension of the Slats. The aircraft starts rattling, on realizing the issue, the flap lever is pulled back to retract them but during retraction of both slats/flaps, Slat #7 remains extended in an unusual position leading to the uncontrollable descent (disruption of airflow over Slat #7). The documentary states at about 5000ft, the upset had caused the hanging Slat #7 to break off the aircraft, making it controllable again.
My Question
Does the sophistication built into present day airliners like the Boeing 737-800, A320neos, A350s, etc. have any protection feature to prevent Extending the Flaps but NOT SLATS during cruise? If they exist, Can they be overridden in the Flight Deck?

Related: How does a flap load relief system work?

There are protections against selecting flaps and slats during cruise in modern airliners.
For example, on the Airbus A380 there is a function to prevent extension of flaps 1 during cruise conditions:

SLATS/FLAPS CRUISE BAULK FUNCTION
In case the FLAPS lever is inadvertently moved from 0 to 1 during cruise, the slats/flaps cruise baulk
function will maintain the slats and flaps in their fully retracted position. In addition, the ECAM will
trigger the F/CTL FLAP LEVER NOT ZERO warning.
The slats/flaps cruise baulk function activates, when the indicated airspeed is greater than the VFE of CONF 1 +2.5 kt, or the aircraft is at an altitude of more than 22 000 ft.
The slats/flaps cruise baulk function is valid only for FLAPS lever position 1. The selection of any other
slats/flaps setting (2, 3, or FULL) will override the cruise baulk function, and allow slats' and flaps'
extension.

(Airbus A380 FCOM 27 Flight Controls - Slats/Flaps)
While this function is only available for flaps in CONF 1, the normal flap load relief system (FLRS) is active for CONF 2, 3, and FULL.
Similarly, on the Boeing 777 any selection out of the UP position is inhibited during cruise conditions:

To protect against inadvertent deployment during cruise, flap and slat extension
from the UP position is inhibited when speed is more than [777-200] 250 [777-200ER and 777-300] 265 [777-200LR, 777F, and 777-300ER] 275 knots or altitude is above approximately 20,000 feet.
If the flap handle is moved out of UP while the flaps are inhibited, LOAD RELIEF
displays.

(Boeing 777 FCOMv2 9.20.19 - Flight Controls - System Description)
For most systems, there is a way to override it in the flight deck because any failure of this system would otherwise create a high risk. Possibly, this could be done by pulling a circuit breaker, but I'm not sure if either of the above systems have a dedicated circuit breaker (not documented in the FCOMs). It might also be possible to override these systems by removing air data sources since the systems are based on knowing the airspeed. This would however create more problems.
The Airbus A350 automatically uses a small amount of differential flaps during cruise to reduce wing bending (for details, see this answer).

Thank You! But I have one question. According to the A380s FCOM (You quoted above) The selection of any other slats/flaps setting (2, 3, or FULL) will override the cruise baulk function, and allow slats' and flaps' extension.. Now the question is, if I set flaps 2 or 3 or even FULL during cruise, can i prevent the extension of slats along with it? In the accident aircraft, I see the circuit breaker prevents extension of slats when flaps are deployed.

@AtheeshThirumalairajan If you really want to, you can do it. On the A380, the slats are driven by an electric motor or the green hydraulic system. The flaps are driven by the green or yellow hydraulic system. The electric motor can almost certainly be disabled with a circuit breaker. If you now disable the green hydraulic system (turn off engine driven pumps and electric pumps), the flaps could still extend while the slats would be unable to move. This is crazy and reckless of course, but technically possible.


text:
Why did the 747-8's freighter version make its first flight before the passenger version?
I have read that the 747-8F entered service in October 2011, almost eight months ahead of the passenger version (747-8I).
Anyone know why this is?
Thanks

The launch customers for the 747-8 were Cargolux and Nippon Cargo Airlines, both air freight operators who wanted the 747-8F. The first orders for the 747-8I didn't materialize until several months later.
Source: Wikipedia


text:
What are some "FAA approved" simulators for home that I can log time on?
I'd assume xplanes on iPhone or Android would not be log-able, but wanted to know if there is a affordable simulator kit for home usage that'd help me logging time for IFR requirements?

Define "affordable", and no "home simulator" would be certified for this.

The only simulators you are able to log hours with are Level-D ZFT simulators and they cost over 10 millions USD. Lead time is 1+ month if you consider it "affordable"

@sahmad Let's not be jerks... What I mean is what is affordable to you may not be my affordable. You may think spending $150,000 is "affordable", whereas I may think that $100 is affordable. "Affordability" is a measure of personal wealth, and since I have no objective basis for yours, I'm asking you what you believe is an "affordable" price range.

@Afe: I've logged time in a Frasca 142, which is an FAA approved flight training device. I'm pretty sure those don't cost anywhere near $10 million. Of course it's technically not a "simulator", but what most people call that doesn't meet the strict definition.

I think the Redbird TD is the cheapest BATD at $7000, but it is for recurrent training, not initial ratings.

Welcome to aviation.SE! When you say "log time", what sort of time are you talking about? PIC time? Training time? Approaches for currency?

@RonBeyer - You are absolutely right. Redbird is a very turnkey solution to FAA certified simulators. AATDs and BATDs can be used for recurrent training. They can also be used for a limited amount of initial training as loggable time. The question would be for what purpose are you logging it. It is not counted in your total time. The limited amount allowed is a substitute for your total time.

You can't log time at home, it's as simple as that. It doesn't matter how much you spend as it's partly the setting.

@GdD - While Parts 61.64, 61.65, 61.109, & 61.129 all have stipulation that use of a flight simulator must be done within a Part 141, 142, 121, or 135 setting or under the supervision of an instructor, the regs do not state a location requirement. Part 61.57(c)(2) does not have such a stipulation for maintaining IFR currency. Also, FAA Form 8710-1 states “The time entered for an FFS, FTD, and/or ATD may be credited towards the total time in the category, class, and instrument time as permitted by the regulations.” So, it is limited in use.

I'm making the assumption that the OP is not going to have an instructor at home when the time would be logged @DeanF.

@GdD - Granted. He can still log time for IFR currency in the sim solo. Although, that time will not be useful towards total time. It also still goes on form 8710-1 in the sim section if done in an FAA approved sim. Though, I agree that it would not be worth the money spent on owning an FAA approved sim since you can rent time in one whenever you need to get recurrent.

Does this answer your question? How do I log simulator time in my logbook as a CFI?

Read this answer from AOPA, especially the last 2 paragraphs.
https://www.aopa.org/news-and-media/all-news/2004/november/flight-training-magazine/since-you-asked

Logging sim time
Dear Rod:
I'm a private pilot just a couple of days from taking my instrument checkride. My CFI and I have had discussions about simulator hours. He logs them but does not include them in total time. I've just closed the Federal Aviation Regulations/Aeronautical Information Manual for the umpteenth time and after reading about maximum of 20 here and a credit of 100 there, I'm confused. Maybe you can explain. What is the point of taking advantage of a sim if the hours logged don't count toward my TT?
Thank you,
Mark
Greetings Mark:
Simulator hours can count as training time, but are not considered "flight time" and can't be logged in the flight time (total time) column of your logbook. They can, however, be logged in the "Flight Simulator" column of your logbook. If your logbook doesn't have such a column, then make one using the letters "FS/FTD" which stand for flight simulator/flight training device.
Regarding simulator time counting toward your total time, consider that the commercial certificate allows 50 hours of simulator time to be counted toward the total aeronautical experience requirement of that rating -- see FAR 61.129(i)(1).
Thus, you could meet the commercial requirements of 250 hours of aeronautical experience with 200 hours of flight time and 50 hours of simulator time.
Even if simulator time didn't count toward the total aeronautical experience requirement of a rating, it might still count as training time. For instance, 2.5 hours of appropriate simulator time can be credited toward the total training time required for the private pilot certificate (FAR 61.109 i).
Now, read the federal aviation regulations carefully before running off and trying to log cross-country time in a simulator. The regs specifically state you must meet certain "flight time" requirements in an airplane. You're not getting away with packing a lunch and sitting in front of a simulator for six hours in hopes of logging this as a cross-country flight to meet the private pilot cross-country requirements.
(Yes, I heard of a case where someone actually tried to do this. Perhaps the ruse unraveled when the FAA examiner asked, "Hey, what kind of airplane is a Combat Flight Sim?")
Also keep in mind that you can only log simulator time when there is an appropriately rated instructor providing dual instruction. In other words, solo time in a simulator can't count toward your private or commercial certificate or instrument rating.
And it goes without saying that a simulator must be approved by the FAA for it to be used as a FS or FTD. (Sorry, but Combat Flight Simulator is only approved for shooting down other simulators, not flight training.)
Rod Machado is a flight instructor, author, educator, and speaker. A pilot for 34 years and a CFI for 30, he has flown more than 8,000 hours and owns a Beech A36 Bonanza. Visit his Web site.

FAA regs on simulators
https://www.faa.gov/about/initiatives/nsp/ac/
List of FAA approved simulators, and the aircraft type simulated
https://www.faa.gov/about/office_org/headquarters_offices/avs/offices/afx/afs/afs800/afs810/media/FAA_Approved_Airplane_ATDs.pdf

To go along with the list of approved ATDs, there is AC 61-136B that provides the information and guidance for the ATDs.

These guys seem to have the hardware that I've seen at my local flight school  https://www.flyelite.ch/en/shop/pro-panel-iii-sep-single-engine-piston/  Never tried it myself.  One thing I found handy when working on my instrument rating was OnTop software, I had a version that let me plug in my Northstar M3 Approach GPS and practice turning actual knobs and buttons at home.  I was very comfortable setting it up and using it when it came for my IFR checkride.  I updated to an Avidyne IFD540 last spring, and practice with IFD100 on an ipad now.

Was pretty bumpy out on Sunday when I was flying, practice real world approaches, was finding it difficult to select things on the touchscreen, or keeping hand where I could turn a knob to select something.  Good thing it was VFR out.

These sims appear reasonably affordable.
https://www.flythissim.com/
"The only simulator manufacturer with FAA approval for a touch screen based Basic Aviation Training Device."
https://flythissim.myshopify.com/collections/all-touchtrainer/products/touchtrainer-standard
$5,400

Isn't that just for training purposes? I imagine there's a difference between approval for training and for logging actual flight time. Your other answer seems to contradict this one.

@Mast The FlyThisSim devices are either basic aviation training devices  (BATD) or advanced aviation training devices (AATD) depending on configuration. They can be used to log flight time. Specifically they can be used to log instrument currency and per §61.65 log training for an instrument rating. "(i) Use of an aviation training device. A maximum of 10 hours of instrument time received in a basic aviation training device or a maximum of 20 hours of instrument time received in an advanced aviation training device may be credited for the instrument time requirements of this section if…"